Electrodeposition of chromium



3,311,548 ELECTRODEPOSITION F CHRGMIUM Henry Brown, Huntington Woods, and Edward A. Romanowski, Troy, Mich, assignors to The Udylite Corporation, Warren, Mich, a corporation of Delaware No Drawing. Filed Feb. 20, 1964, Ser. No. 346,095 16 Claims. (Cl. 204S1) This invention relates to improvements in the electrodeposition of chromium from aqueous acidic hexavalent chromium solutions. More particularly, it relates to the electrodeposition of chromium plate of improved covering power, which is accomplished by the use of certain aliphatic dicarboxylic acids as additives to acidic hexavalent chromium electroplating baths. These new organic additives to acidic hexavalent chromium plating baths function as cooperating activators with minimum concentrations of catalyst ion (the inorganic sulfate anion) to make possible chromium plate of extremely good covering power without complicating the operation of the chromium plating bath.

In order to electrodeposit chromium from acidic hexavalent chromium solutions, that is, chromic acid solutions, it was early established that it was necessary to have present in the bath small concentrations of certain anions such as the sulfate ion, and included with the sulfate ion, fluoride ions and other ions were also named as effective catalyst anions. However, many anions were termed catalyst anions that merely aided the effect of low concentrations of sulfate ions. It really appears that the sulfate anion is the only true catalyst anion and that the other mentioned anions have auxiliary effects.

The standard or conventional chromium bath employing only the sulfate anion as catalyst and used for plating on nickel, ferrous surfaces, yellow brass, or copper has been the 100 to 1 ratio of .chromic acid anhydride (CrO to sulfate ion. Thus in a 200 gram/liter chromic acid bath, 2 grams per liter of sulfate ion would be used, and in a 400 grams/liter chromic acid bath, 4 grams/liter of sulfate ion would be used. If one uses instead a ratio of 200 to l of CrO to S0,, and plates on top of nickel, copper, yellow brass or steel, only an iridescent (rainbow) non-metallic chromium chromate film is obtained instead of chromium plate if dead entry into the chromium plating bath is used. Furthermore, if chromium plating is attempted on top of a freshly plated bright nickel surface, from an aqueous solution containing, for example, 40 grams/ liter (45 oz./ gal.) of chromic acid and saturated with strontium sulfate at, for example, about 50S2 C. (approx. 125 P.) which yields a ratio of chromic acid to dissolved sulfate ion of about 160 to 1, and using dead entry into this chromic acid bath (that is, the direct plating current is turned on only after the nickel plated panel is immersed in the bath), n0 chromium plate is obtained on the nickel surface, only a non-metallic iridescent film of a basic chromic chromate results. However, if live entry is used, that is, the electrical connection is made before the nickel plated panel is immersed into the chromic acid bath, and the plating thus is started just as the panel or work goes into the bath, then a chromium plate is obtained on the nickel. With white brass plate (80% zinc and 20% copper alloy plate), even with dead entry into the chromium plating bath of 200 to 1 ratio of chromic acid to sulfate chromium plate is obtained on top of the white brass plate. It is believed that the white brass surface is less passive than the nickel surface and that the sulfate ion functions in part as catalyst and in part as an activator of passive surfaces such as nickel surfaces. The longer the nickel surface is allowed to stay in contact with the chromic acid solution with no current on, and the more concentrated the chromic acid,

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the poorer will be the chromium coverage. If the sulfate ion is increased much beyond the 100 to 1 ratio, for example, to to 1 ratio of chromic acid to sulfate, then the chromium coverage gets poorer and poorer and finally with a large excess of sulfate ion no chromium plate is obtained, and merely trivalent chromium is formed at the cathode.

It has now been found that by the use of certain aliphatic dicarboxylic acids characterized by Formula A below, in conjunction with low concentrations of the catalyst sulfate anion, it is possible to obtain extremely good chromium coverage with chromic acid baths of about 150 to 1, 200 to 1, and 300 to 1 ratios of chromic acid to sulfate. Even 400 to 1 ratios of chromic acid to sulfate are helped by the dicarboxylic acids of Formula A HOOC(CH COOH (A) where x=an integer from 1 to 8 inclusive. The preferred compounds represented by the above Formula A are succinic acid, glutaric, adipic, azelaic and sebacic acids. Suberic acid is quite expensive at present and malonic acid tends to slowly decarboxylate when the baths are kept hot (140 F.), however, it does give very good results. All of these dicarboxylic acids represented by the formula above are quite stable to oxidation by the chromic acid but are not perfectly stable to the more powerful anodic oxidation, though far more stable than tartaric acid in the chromium plating baths.

The Formula A dicarboxylic acids without the sulfate anion do not make possible the electrodeposition of chromium from pure chromic acid solutions. However, in conjunction with small concentrations of sulfate anion they act as uncritical activators, that is, the sulfate anion is needed but only in optimum concentration for its catalytic effect and not for any special activating effect.

It is not new to use carboxylic acids in acidic hexavalent chromium plating solutions, however, such carboxylic acids as acetic acid (or acetates) strongly attack the lead and lead alloy anodes that are used in chromic acid plating baths, and these anodes during use are corroded away at a rapid rate. This is also true for chloro acetic acids and trifiuoro acetic acid. Du Rose in US. 2,784,153 (March 5, 1957) has found that the presence of cobalt ions in the chromic acid bath decreases the attack by the acetates, however, the Formula A dicarboxylic acids do not corrode the lead and lead alloy anodes nearly as fast as acetate ions or acetic acid, including mono-, diand trichloroacetic acids (Murray, US. Patent 2,279,830, issued Apr. 14, 1942), and trifiuoroacetic acid, and this improvement represents a very important and unexpected advance in the art. The corrosive attack on the lead and lead alloy anodes by the Formula A dicarboxylic acids is no greater than that which occurs in the widely used chromium plating baths employing sulfate plus fluosilicate catalyst ions. Acetic acid, and the mono-, diand trichloroacetic acids and trifluoroacetic acids used in the same concentrations in the chromium plating bath as the Formula A dicarboxylic acids attack the lead and lead alloy anodes at least five times faster.

Dicarboxylic acids such as tartaric, and phthalic acids are rapidly oxidized away by the chromic acid bath chromium ions formed at the cathode during the electrodeposition of the chromium metal. If most of this trivallent chromium is not re-oxidized, the chromium bath will become almost inoperative. The powerful oxidizing agent, lead dioxide, forms on the anodes during electrolysis, and this oxidizing agent re-oxidizes the trivalent chromium to the hexavalent form.

The acidic hexavalent chromium plating baths may be made up from straight chromic acid anhydride or chromic acid, and from mixtures with dichromates, chromates and polychromates. It is generally preferred to use straight chromic acid or chromic acid anhydride. The source of the sulfate ion may he sulfuric acid or one of its soluble salts, or from strontium sulfate in saturation concentrations, with or without added strontium ions from another source such as strontium chromate or dichromate or carbonate.

Below are given examples of baths of this invention. These baths may be modified by adding small concentrations of fluoride, fluosilicate, fluoborate, fluotitanate, fiuoaluminate or iluozirconate ions, or boric acid or saturated concentrations of thorium fluoride, thorium fluoborate or fiuozirconate, uranium tetrafluoborate, uranium tetrafluozirconate. Also other organic activators such as, for example, trifluoromethyl sulfonic acid, or 2-hydroperfluoroethyl-l-phosphonic acid (HCF CF PO(OH) can be used together with the formula A dicarboxylic acids. For example, with 2-6 grams/liter of trifiuoromethyl sulfonic acid and/or the 2-hydroperfluoroet-hyl-1- phosphonic acid, only 0.1 g./l of the Formula A dicarboxylic acids will show improvement in activation. As high as about 20 grams/ liter of the Formula A dicarboxylic acids can be used, though in general the optimum concentrations are from about 0.5 to about 10 grams/ liter. The optimum concentration to use depends on the purity and passivity of the nickel plate, and the ratio of the concentration of the chromic acid to the sulfate anion. In most cases the optimum concentration of the Formula A dicarboxylic acid is in the range of 1 to 10 grams/liter. With ratios of chromic acid to sulfate of about 100 to 1 or 90 to 1 very little of these activators is needed but then the chromium coverage is not as great as when the ratio of chromic acid is about 300 to l or 200 to l, or 150 to 1.

EXAMPLE I 200-400 grams/liter CrO 1-1.5 grams/liter S anion 3-8 grams/liter of succinic, adipic, or azelaic acid or sebacic acid 0-3 grams/liter perfluoro p-ethyl cyclohexyl sulfonic acid Temp. 40-55 C. (105-130 F.)

EXAMPLE II 300-400 grams/liter CrO Saturation concentrations of SrSO (excess present) 0-50 grams/liter of strontium chromate 2-10 grams/liter of succinic, adipic, glutaric, or azelaic acid 0-30 grams/liter boric acid 0-5 grams/liter perfluoro p-ethyl cyclohexyl sulfonic acid (Instead of the strontium salts above, sulfuric acid or other source of sulfate anion can be used to give about 300 to 1 to about 140 to 1 ratio of chromic acid to sulfate.)

Temp. 40-60 C. (105-140 F.)

EXAMPLE III 300-400 grams/liter CrO Saturation concentration of SrSO (excess present) Saturation concentration of ThF (excess present) Boric acid grams/liter to saturation 1-8 grams/liter of succinic, adipic, glutaric, azelaic or sebacic acid 0-5 grams/liter perfluoro p-ethyl cyclohexyl sulfonic acid EXAMPLE 1v 300-400 grams/ liter CrO Saturation concentration of SrSO (excess present) Saturation concentration of ThF; (excess present) 'Boric acid 10 grams/liter to saturation 1-8 grams/liter of adipic, glutaric, azelaic and sebacic acids, especially the last two are less soluble than succinic acid and appear to be more stable in the chromium plating bath.

With the higher molecular Weight dicarboxylic acids such as azelaic and sebacic acids there was evidence that less decomposition at the lead dioxide covered anode took place in the presence of the perfiuoro wetting agents such as perfiuoro p-ethyl cyclohexyl sulfonic acid. It appears that the strong perfluoro anionic wetting agent by concentrating around the anode decreases the rate at which the dicarboxylic acids reach the anode surface.

What is claimed is:

1. A method of electrodepositing chromium which comprises electrolyzing an aqueous acidic hexavalent chromium plating solution containing a ratio of CrO to sulfate ion greater than about 150 to 1 and less than about 400 to 1 and containing therein dissolved about 0.1 gram/liter to about 20 grams/ liter of a dicarboxylic acid characterized 'by the following formula HOOC- (CH -COOH said said

said

- dicarboxylic acid is adipic acid.

5. A method in accordance with claim 1 wherein dicarboxylic acid is azelaic acid.

6. A method in accordance with claim 1 wherein dicarboxylic acid is sebacic acid.

7. A method in accordance with claim 1 wherein said chromium plating solution is chromic acid in a concentration of about to about 500 grams/liter.

8. A method in accordance with claim 1 wherein said sulfate ion is derived from saturationconcentrations of strontium sulfate in said acidic hexavalent chromium plating bath.

9. A bath for the electrodeposition of chromium comprising an aqueous acidic hexavalent chromium plating solution containing a ratio of CrO to sulfate ion greater than about to 1 and less than about 400 to 1 and containing dissolved therein about 0.1 gram/liter to about 20 grams/liter of a dicarboxylic acid characterized by the following formula where x is an integer from 1 to 8 inclusive.

10. A bath as claimed in claim 9 and wherein said dicarboxylic acid is succinic acid.

11. A bat-h as claimed in claim 9 and wherein said dicarboxylic acid is glutaric acid.

12. A bath as claimed in claim 9 and wherein said dicarboxylic acid is adipic acid.

13. A bath as claimed in claim 9 and wherein said dicarboxylic acid is azelaic acid.

14. A bath as claimed in claim 9 and wherein said dicarboxylic acid is sebacic acid.

15. A bath in accordance with claim 9 wherein said hexavalent chromium plating solution is chromic acid in a concentration of about 100 to about 500 grams/liter.

16. A bath in accordance with claim 9 wherein said sulfate ion is derived from saturation concentrations of strontium sulfate in said acidic hexavalent chromium plating bath. 0

said

(References on following page) 6 References Cited by the Examiner 2,640,022 5/ 1953 Stareck 204,51 UNITED STATES A NTS 3,129,149 4/1964 Johnson 20451 X 2,063,197 12/1936 Schenidewind 20451 JOHN H. MACK, Primary Examiner.

2,517,441 8/1950 Raab 20451 5 G. KAPLAN, Assistant Examiner. 

1. A METHOD OF ELECTRODEPOSITING CHROMIUM WHICH COMPRISES ELECTROLYZING AN AQUEOUS ACIDIC HEXAVALENT CHROMIUM PLATING SOLUTION CONTAINING A RATIO OF CRO3 TO SULFATE ION GREATER THAN ABOUT 150 TO 1 AND LESS THAN ABOUT 400 TO 1 AND CONTAINING THEREIN DISSOLVED ABOUT 0.1 GRAM/LITER TO ABOUT 20 GRAMS/LITER OF A DICARBOXYLIC ACID CHARACTERIZED BY THE FOLLOWING FORMULA 